// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Weak pointers are pointers to an object that do not affect its lifetime,
// and which may be invalidated (i.e. reset to nullptr) by the object, or its
// owner, at any time, most commonly when the object is about to be deleted.

// Weak pointers are useful when an object needs to be accessed safely by one
// or more objects other than its owner, and those callers can cope with the
// object vanishing and e.g. tasks posted to it being silently dropped.
// Reference-counting such an object would complicate the ownership graph and
// make it harder to reason about the object's lifetime.

// EXAMPLE:
//
//  class Controller {
//   public:
//    Controller() : weak_factory_(this) {}
//    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
//    void WorkComplete(const Result& result) { ... }
//   private:
//    // Member variables should appear before the WeakPtrFactory, to ensure
//    // that any WeakPtrs to Controller are invalidated before its members
//    // variable's destructors are executed, rendering them invalid.
//    WeakPtrFactory<Controller> weak_factory_;
//  };
//
//  class Worker {
//   public:
//    static void StartNew(const WeakPtr<Controller>& controller) {
//      Worker* worker = new Worker(controller);
//      // Kick off asynchronous processing...
//    }
//   private:
//    Worker(const WeakPtr<Controller>& controller)
//        : controller_(controller) {}
//    void DidCompleteAsynchronousProcessing(const Result& result) {
//      if (controller_)
//        controller_->WorkComplete(result);
//    }
//    WeakPtr<Controller> controller_;
//  };
//
// With this implementation a caller may use SpawnWorker() to dispatch multiple
// Workers and subsequently delete the Controller, without waiting for all
// Workers to have completed.

// ------------------------- IMPORTANT: Thread-safety -------------------------

// Weak pointers may be passed safely between threads, but must always be
// dereferenced and invalidated on the same SequencedTaskRunner otherwise
// checking the pointer would be racey.
//
// To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
// is dereferenced, the factory and its WeakPtrs become bound to the calling
// thread or current SequencedWorkerPool token, and cannot be dereferenced or
// invalidated on any other task runner. Bound WeakPtrs can still be handed
// off to other task runners, e.g. to use to post tasks back to object on the
// bound sequence.
//
// If all WeakPtr objects are destroyed or invalidated then the factory is
// unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
// destroyed, or new WeakPtr objects may be used, from a different sequence.
//
// Thus, at least one WeakPtr object must exist and have been dereferenced on
// the correct thread to enforce that other WeakPtr objects will enforce they
// are used on the desired thread.

#ifndef BASE_MEMORY_WEAK_PTR_H_
#define BASE_MEMORY_WEAK_PTR_H_

#include <cstddef>
#include <type_traits>

#include "base/base_export.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/sequence_checker.h"

namespace base {

template <typename T>
class SupportsWeakPtr;
template <typename T>
class WeakPtr;

namespace internal {
    // These classes are part of the WeakPtr implementation.
    // DO NOT USE THESE CLASSES DIRECTLY YOURSELF.

    class BASE_EXPORT WeakReference {
    public:
        // Although Flag is bound to a specific SequencedTaskRunner, it may be
        // deleted from another via base::WeakPtr::~WeakPtr().
        class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> {
        public:
            Flag();

            void Invalidate();
            bool IsValid() const;

        private:
            friend class base::RefCountedThreadSafe<Flag>;

            ~Flag();

            SequenceChecker sequence_checker_;
            bool is_valid_;
        };

        WeakReference();
        explicit WeakReference(const Flag* flag);
        ~WeakReference();

        WeakReference(WeakReference&& other);
        WeakReference(const WeakReference& other);
        //  WeakReference& operator=(WeakReference&& other) = default;
        WeakReference& operator=(const WeakReference& other) = default;

        bool is_valid() const;

    private:
        scoped_refptr<const Flag> flag_;
    };

    class BASE_EXPORT WeakReferenceOwner {
    public:
        WeakReferenceOwner();
        ~WeakReferenceOwner();

        WeakReference GetRef() const;

        bool HasRefs() const
        {
            return flag_.get() && !flag_->HasOneRef();
        }

        void Invalidate();

    private:
        mutable scoped_refptr<WeakReference::Flag> flag_;
    };

    // This class simplifies the implementation of WeakPtr's type conversion
    // constructor by avoiding the need for a public accessor for ref_.  A
    // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
    // base class gives us a way to access ref_ in a protected fashion.
    class BASE_EXPORT WeakPtrBase {
    public:
        WeakPtrBase();
        ~WeakPtrBase();

        WeakPtrBase(const WeakPtrBase& other) = default;
        //  WeakPtrBase(WeakPtrBase&& other) = default;
        WeakPtrBase& operator=(const WeakPtrBase& other) = default;
        //  WeakPtrBase& operator=(WeakPtrBase&& other) = default;

    protected:
        explicit WeakPtrBase(const WeakReference& ref);

        WeakReference ref_;
    };

    // This class provides a common implementation of common functions that would
    // otherwise get instantiated separately for each distinct instantiation of
    // SupportsWeakPtr<>.
    class SupportsWeakPtrBase {
    public:
        // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
        // conversion will only compile if there is exists a Base which inherits
        // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
        // function that makes calling this easier.
        template <typename Derived>
        static WeakPtr<Derived> StaticAsWeakPtr(Derived* t)
        {
            static_assert(
                std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
                "AsWeakPtr argument must inherit from SupportsWeakPtr");
            return AsWeakPtrImpl<Derived>(t, *t);
        }

    private:
        // This template function uses type inference to find a Base of Derived
        // which is an instance of SupportsWeakPtr<Base>. We can then safely
        // static_cast the Base* to a Derived*.
        template <typename Derived, typename Base>
        static WeakPtr<Derived> AsWeakPtrImpl(
            Derived* t, const SupportsWeakPtr<Base>&)
        {
            WeakPtr<Base> ptr = t->Base::AsWeakPtr();
            return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_));
        }
    };

} // namespace internal

template <typename T>
class WeakPtrFactory;

// The WeakPtr class holds a weak reference to |T*|.
//
// This class is designed to be used like a normal pointer.  You should always
// null-test an object of this class before using it or invoking a method that
// may result in the underlying object being destroyed.
//
// EXAMPLE:
//
//   class Foo { ... };
//   WeakPtr<Foo> foo;
//   if (foo)
//     foo->method();
//
template <typename T>
class WeakPtr : public internal::WeakPtrBase {
public:
    WeakPtr()
        : ptr_(nullptr)
    {
    }

    WeakPtr(std::nullptr_t)
        : ptr_(nullptr)
    {
    }

    // Allow conversion from U to T provided U "is a" T. Note that this
    // is separate from the (implicit) copy and move constructors.
    template <typename U>
    WeakPtr(const WeakPtr<U>& other)
        : WeakPtrBase(other)
        , ptr_(other.ptr_)
    {
    }
    template <typename U>
    WeakPtr(WeakPtr<U>&& other)
        : WeakPtrBase(std::move(other))
        , ptr_(other.ptr_)
    {
    }

    T* get() const { return ref_.is_valid() ? ptr_ : nullptr; }

    T& operator*() const
    {
        DCHECK(get() != nullptr);
        return *get();
    }
    T* operator->() const
    {
        DCHECK(get() != nullptr);
        return get();
    }

    void reset()
    {
        ref_ = internal::WeakReference();
        ptr_ = nullptr;
    }

    // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
    explicit operator bool() const { return get() != nullptr; }

private:
    friend class internal::SupportsWeakPtrBase;
    template <typename U>
    friend class WeakPtr;
    friend class SupportsWeakPtr<T>;
    friend class WeakPtrFactory<T>;

    WeakPtr(const internal::WeakReference& ref, T* ptr)
        : WeakPtrBase(ref)
        , ptr_(ptr)
    {
    }

    // This pointer is only valid when ref_.is_valid() is true.  Otherwise, its
    // value is undefined (as opposed to nullptr).
    T* ptr_;
};

// Allow callers to compare WeakPtrs against nullptr to test validity.
template <class T>
bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t)
{
    return !(weak_ptr == nullptr);
}
template <class T>
bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr)
{
    return weak_ptr != nullptr;
}
template <class T>
bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t)
{
    return weak_ptr.get() == nullptr;
}
template <class T>
bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr)
{
    return weak_ptr == nullptr;
}

// A class may be composed of a WeakPtrFactory and thereby
// control how it exposes weak pointers to itself.  This is helpful if you only
// need weak pointers within the implementation of a class.  This class is also
// useful when working with primitive types.  For example, you could have a
// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
template <class T>
class WeakPtrFactory {
public:
    explicit WeakPtrFactory(T* ptr)
        : ptr_(ptr)
    {
    }

    ~WeakPtrFactory() { ptr_ = nullptr; }

    WeakPtr<T> GetWeakPtr()
    {
        DCHECK(ptr_);
        return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
    }

    // Call this method to invalidate all existing weak pointers.
    void InvalidateWeakPtrs()
    {
        DCHECK(ptr_);
        weak_reference_owner_.Invalidate();
    }

    // Call this method to determine if any weak pointers exist.
    bool HasWeakPtrs() const
    {
        DCHECK(ptr_);
        return weak_reference_owner_.HasRefs();
    }

private:
    internal::WeakReferenceOwner weak_reference_owner_;
    T* ptr_;
    DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
};

// A class may extend from SupportsWeakPtr to let others take weak pointers to
// it. This avoids the class itself implementing boilerplate to dispense weak
// pointers.  However, since SupportsWeakPtr's destructor won't invalidate
// weak pointers to the class until after the derived class' members have been
// destroyed, its use can lead to subtle use-after-destroy issues.
template <class T>
class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
public:
    SupportsWeakPtr() { }

    WeakPtr<T> AsWeakPtr()
    {
        return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
    }

protected:
    ~SupportsWeakPtr() { }

private:
    internal::WeakReferenceOwner weak_reference_owner_;
    DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
};

// Helper function that uses type deduction to safely return a WeakPtr<Derived>
// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
// extends a Base that extends SupportsWeakPtr<Base>.
//
// EXAMPLE:
//   class Base : public base::SupportsWeakPtr<Producer> {};
//   class Derived : public Base {};
//
//   Derived derived;
//   base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
//
// Note that the following doesn't work (invalid type conversion) since
// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
// the caller.
//
//   base::WeakPtr<Derived> ptr = derived.AsWeakPtr();  // Fails.

template <typename Derived>
WeakPtr<Derived> AsWeakPtr(Derived* t)
{
    return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
}

} // namespace base

#endif // BASE_MEMORY_WEAK_PTR_H_
